Check engine light on: what to do

The check engine light says only one thing: the control unit has recorded a fault. It says neither which one, nor whether it is serious. Here is how to find out, and what to do in each case.

Steady or flashing: the only distinction that matters

A steady check engine light signals a recorded, confirmed fault. The engine may or may not have gone into limp mode; either way you can drive carefully to somewhere you can read the code. Nothing requires you to stop on the hard shoulder.

A flashing check engine light is a different message: a misfire is happening right now. Unburnt fuel is reaching the catalytic converter, where it burns at a temperature it was never built to withstand — a few dozen kilometres are enough to destroy it, and it costs more than the original repair. Ease off immediately, stop as soon as it is safe, and do not set off again if the flashing resumes.

Do not confuse it with the red lights — temperature, oil pressure, battery. Those are not the check engine light, and they demand an immediate stop.

What the light does not tell you

Behind the same orange pictogram there are more than seven hundred standardised faults, from a loose fuel cap (P0442) to a dead catalytic converter (P0420), from an ageing sensor (P0133) to the sensor that stops the engine starting (P0335). Some have no consequence; others get expensive if ignored.

The dashboard does not tell them apart. The only way to know is to read the code the control unit recorded.

Read the code yourself

Every car sold in Europe since 2001 for petrol and 2004 for diesel has an OBD2 diagnostic socket, usually under the steering wheel, to the left or above the pedals. An adapter costing a few euros plugs into it, and an app on your phone reads what the control unit recorded.

Ignition on, the whole thing takes two minutes. You get the code — P0420, say —, its description, and in Klaro its severity and what to do next. You also see the “pending” codes: faults seen once, not yet confirmed, which often announce what is coming.

Even if you leave the repair to the garage, arriving with the code changes the conversation: you will know whether the quote matches the problem.

The codes you meet most often

• P0420 — the catalytic converter no longer converts enough, or a sensor says so
• P0300 — misfires on several cylinders
• P0171 — mixture too lean, most often an air leak
• P0401 — EGR valve caked up, especially on diesels
• P0442 and P0455 — leak in the fuel vapour system, often the cap
• P0335 — crankshaft sensor, the car stalls or will not start
• P0380 — glow plugs, on diesels

Every code has its page: what it means, its symptoms, its causes in order of likelihood, and what the repair costs.

Drive or not: the three cases

Steady light, no symptom: the car drives normally, nothing is urgent. Read the code, and have it checked within the week. This is the most common case.

Steady light with symptoms — loss of power, limp mode, unstable idle, unusual noise: keep trips to the essential and get to a workshop soon. Limp mode protects the engine, but not indefinitely.

Flashing light: stop. If the flashing stops at idle and does not resume when driving gently, make for the nearest garage at low revs. If it resumes, get towed: the converter is worth more than the tow truck.

The light went off by itself

A fault that does not recur for several drive cycles switches the light off on its own. The code stays recorded for a while in the control unit's history — and the fault has a good chance of coming back, especially if it depends on temperature or humidity.

Read it while it is there, and write it down. An intermittent fault is easier to diagnose when you already know what it reports.

Do not clear before you understand

Clearing the code switches the light off. It repairs nothing, and it also erases the data recorded at the moment of the fault — revs, temperature, load — that help make sense of it. Read first, note it, and keep clearing for after the repair: Clearing a fault code yourself.

What you need

An OBD2 adapter — see Which adapter to choose — and Klaro. The code appears in plain words, with its severity and what to do next, with no account and no Internet connection.